OPS-lasers typically include a single “OPS-chip” which includes a multilayer mirror-structure surmounted by an epitaxially grown, multilayer, semiconductor gain structure. The gain structure generally includes quantum-well (QW) or active layers spaced apart by a half-wavelength at the peak gain wavelength by pump-light absorbing spacer layers. The OPS-chip is typically solder-bonded, “mirror-structure down”, on a heat sink of some kind. The gain-structure is usually optically pumped by radiation from a diode-laser bar package. The mirror structure provides one mirror (either a fold-mirror or an end-mirror) of a laser-resonator. The laser-resonator usually includes a birefringent filter (BRF) for selecting a particular fundamental wavelength from a relatively broad gain bandwidth of the active layers. This BRF also establishes the polarization-orientation of the circulating and output radiation of the laser-resonator.
OPS-lasers have found use as sources of high-quality, continuous-wave (CW) laser beams with relatively high power. By varying the composition of the active layers and spacer layers, the fundamental lasing wavelength can be selected in a relatively large range of wavelengths from visible to near infrared (NIR). Intra cavity (IC) frequency conversion of the fundamental wavelength to harmonic wavelengths (frequency multiplication) or sum-component wavelengths (optical parametric conversion or frequency division) further extends the available wavelength range. This frequency conversion is effected by one or more optically nonlinear crystals in the laser-resonator The BRF in frequency-converted OPS-lasers fixes the fundamental radiation at a wavelength for which an optically nonlinear crystal is phase-matched.
U.S. Pat. No. 6,097,742, and U.S. Pat. No. 7,447,245, include detailed descriptions of OPS lasers delivering fundamental radiation; OPS lasers including IC harmonic generation; and OPS lasers including IC optical parametric oscillation (OPO). These patents are assigned to the assignee of the present invention and the complete disclosure thereof is hereby incorporated herein by reference.
The power available from an OPS-Chip by increasing pump-power is eventually limited by a phenomenon known to practitioners of the art as “thermal roll-off”. This is caused by generation of free electrons which increase absorption in the chip, which generates more free-electrons, and so on, leading to a complete loss of power. The onset of thermal roll-off can be extended by suitable bonding and cooling techniques but not avoided altogether. If more out power is required, then the only approach available has been to include one or more additional OPS chips in a (multi-chip) resonator.
Early versions of multi-chip resonators were limited in performance because of variations in the lasing-mode path caused by thermal distortion of one or more of the chips. This would cause the mode on one chip to shift from coincidence with the pump light directed onto that chip thereby reducing the gain available from that chip. This was mitigated by further efforts in chip bonding, and in particular by designing resonators with 1:1 imaging relays for directing the mode onto the chips. One such arrangement is described in detail in U.S. Pat. No. 7,408,970, which is also assigned to the assignee of the present invention, and the complete disclosure of which is also hereby incorporated herein by reference.
Adding such relay optics (mirrors) adds significantly to the cost of forming a multi-chip OPS laser-resonator. As relay mirrors are never perfect, the addition of relay optics also adds to round-trip losses in the resonator reducing available output power. Further, the lasing mode is incident on the chips at non-normal incidence which reduces available power due to interference (fringe forming) effects between the incident and reflected modes on the chip which cause spatial hole-burning. There is a need for a multi-chip resonator design that avoids these problems of prior-art multi-chip OPS-lasers.